For something that emits no light that we can detect, black holes love to blanket themselves in radiation.
In fact, some of the brightest light in the Universe comes from supermassive black holes. Well, not really black holes themselves; it is the material that surrounds them as they actively suck up large amounts of matter from their immediate surroundings.
Among the brightest of these maelstroms of swirling hot matter are galaxies known as blazars. Not only do they glow with the heat of a swirling mantle, but they also channel matter into “blazing” beams that shoot through the cosmos, emitting electromagnetic radiation at energies that are difficult to comprehend.
Scientists have finally discovered the mechanism producing the incredible high-energy light that has been beaming at us for billions of years: shocks in the black hole’s jets that increase the speed of particles to breathtaking speeds.
“It’s a 40-year-old mystery that we have solved,” says astronomer Yannis Liodakis of the Finnish Center for Astronomy with ESO (FINCA). “We finally had all the pieces of the puzzle, and the picture they made was clear.”
Most galaxies in the Universe are built around a supermassive black hole. These mind-bogglingly large objects sit at the center of the galaxy, sometimes doing very little (like Sagittarius A*, the black hole at the heart of the Milky Way) and sometimes doing a lot.
This activity consists of accrete matter. A vast cloud gathers into an equatorial disk around the black hole, encircling it like water around a drain. The frictional and gravitational interactions at play in the outer space surrounding a black hole cause this material to heat up and glow over a range of wavelengths. It is one of the light sources of a black hole.
The other – the one at play in the blazars – are twin jets of matter launched from the polar regions outside the black hole, perpendicular to the disk. These jets are believed to be materials from the inner edge of the disk which, rather than falling towards the black hole, are accelerated along the outer magnetic field lines towards the poles, where they are launched at very high speeds, close of the speed of light. .
For a galaxy to be classified as a blazar, these jets must be aimed almost directly at the viewer. It’s us on Earth. Thanks to the particles’ extreme acceleration, they radiate light across the entire electromagnetic spectrum, including high-energy gamma and X-rays.
Exactly how this jet accelerates particles to such high speeds has been a giant cosmic question mark for decades. But now a powerful new X-ray telescope called the Imaging X-ray Polarimetry Explorer (IXPE), launched in December 2021, has given scientists the key to solving the mystery. It is the first space telescope to reveal the orientation, or polarization, of X-rays.
“The first measurements of X-ray polarization of this class of sources allowed, for the first time, a direct comparison with models developed from the observation of other frequencies of light, from radio to gamma rays at very high energy,” says astronomer Immacolata Donnarumma of the Italian Space Agency.
IXPE was shot at the brightest high-energy object in our sky, a blazar called Markarian 501, located 460 million light-years away in the constellation of Hercules. For a total of six days in March 2022, the telescope collected data on the X-ray light emitted by the blazar jet.
At the same time, other observatories were measuring light from other wavelength ranges, from radio to optical, which were previously the only data available for Markarian 501.
The team quickly noticed a curious difference in the X-ray light. Its orientation was significantly more twisted, or polarized, than low-energy wavelengths. And optical light was more polarized than radio frequencies.
However, the direction of the polarization was the same for all wavelengths and aligned with the direction of the jet. The team found this to be consistent with models in which shocks in the jets produce shock waves that provide additional acceleration along the length of the jet. Closer to the shock, this acceleration is at its maximum, producing X-ray radiation. Further along the jet, the particles lose energy, producing an optical then radio emission of lower energy, with a weaker polarization.
“As the shock wave passes through the region, the magnetic field becomes stronger and the energy of the particles increases,” says astronomer Alan Marscher of Boston University. “The energy comes from the motion energy of the material that produces the shock wave.”
It’s unclear what creates the shocks, but one possible mechanism is faster material in the jet catching up with slower clumps, leading to collisions. Future research could help confirm this hypothesis.
Given that blazars are among the most powerful particle accelerators in the Universe and one of the best laboratories for understanding extreme physics, this research marks a pretty important piece of the puzzle.
Future research will continue to observe Markarian 501 and turn the IXPE to other blazars to see if a similar bias can be detected.
The research has been published in natural astronomy.
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